Method and arrangement to estimate transmission channel characteristics

ABSTRACT

A channel characteristic of a transmission channel between a remote terminal and a central office is estimated by collecting information in the remote terminal, transferring this information from the remote terminal to the central office and matching the received information at the central office with a channel and remote terminal simulation model. The values of parameters of the latter simulation model, which define the channel characteristic to be estimated, are determined as a result of the matching process. The information collected by the remote terminal typically is information that is automatically collected by the remote terminal for operational purposes, such as equalizer setting and bit allocation.

This is a continuation of U.S. application Ser. No. 10/423,975 filedApr. 28, 2003, which is a continuation of U.S. application Ser. No.09/523,581 filed Mar. 10, 2000, now U.S. Pat. No. 6,606,719, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method to estimate a transmissionchannel characteristic, a transmission channel characteristic estimatingarrangement able to perform this method, and a related remote terminal.

Such a method to estimate a transmission channel characteristic andrelated equipment are already known in the art, e.g. from the U.S. Pat.No. 4,105,995, entitled ‘Digitally Controlled Transmission ImpairmentMeasurement Apparatus’. Typically, the operator needs knowledge ofcertain channel characteristics of a transmission channel between acentral office and a remote terminal to be able to guarantee a certainservice to a subscriber that has installed the remote terminal. Thepresence of line imperfections such as bridged taps, line attenuation,ageing effects, disturbers like radio frequency interference, and so on,has to be estimated by the operator before a certain quality of servicecan be guaranteed. To estimate all basic parameters necessary tocharacterise a transmission channel for its ability to carry datatraffic, U.S. Pat. No. 4,105,995 describes a portable microprocessorcontrolled apparatus that measures specified parameters for a telephonechannel. The known apparatus for channel characteristic estimationeither consists of two units to be connected respectively to both endsof the transmission channel, or of a single unit to be connected to asingle end of the transmission channel. The latter embodiment of theknown channel characteristic estimating apparatus requires manualloop-back at the end of the transmission channel whereto the apparatusis not coupled. Summarising, U.S. Pat. No. 4,105,995 describes dedicatedtransmission channel test equipment enabling off-line measurement ofchannel characteristics. To apply the known technique, transmission ofuser data over the transmission channel has to be interrupted and testequipment has to be connected to the transmission channel either at oneor at both sides of the channel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forestimation of a transmission channel characteristic and relatedequipment similar to the above described one, but which allows bothon-line and off-line channel characteristic estimation without thenecessity to connect dedicated test equipment to the transmissionchannel.

According to the invention, this object is achieved by the method toestimate a transmission channel characteristic, the transmission channelcharacteristic estimating arrangement, and the remote terminal.

Indeed, the remote terminal itself collects all information required toestimate the transmission channel characteristic of interest and sendsthis information back to the central office wherein the information ismatched with a simulation model of the transmission channel and remoteterminal. The information collected by the remote terminal can be fedback in a multiplexed way simultaneously with user data so that the datatraffic over the transmission channel does not have to be interrupted.The simulation model used in the central office for the matching processtypically contains a number of parameters, e.g. the location along thetransmission line of a bridged tap, corresponding to the transmissionchannel characteristics of interest. During the matching process, thevalues of these parameters are determined.

It is to be noticed that the term ‘comprising’, used in the claims,should not be interpreted as being limitative to the means listedthereafter. Thus, the scope of the expression ‘a device comprising meansA and B’ should not be limited to devices consisting only of componentsA and B. It means that with respect to the present invention, the onlyrelevant components of the device are A and B.

Similarly, it is to be noticed that the term ‘coupled’, also used in theclaims, should not be interpreted as being limitative to directconnections only. Thus, the scope of the expression ‘a device A coupledto a device B’ should not be limited to devices or systems wherein anoutput of device A is directly connected to an input of device B. Itmeans that there exists a path between an output of A and an input of Bwhich may be a path including other devices or means.

Indeed, the remote terminal of an Asymmetric Digital Subscriber Line(ADSL) system operating in accordance with ANSI's T1.413 standard, issue2 for instance automatically collects information concerning theattenuation of a set of carriers with equidistant frequencies as can bededuced from paragraph 9.9.8.1 of the draft edition of issue 2 of ANSI'sT1E1.413 standard, entitled ‘Standards Project for Interfaces Relatingto Carrier to Customer Connection of Asymmetric Digital Subscriber Line(ADSL) Equipment. This information is collected for use in the bit rateselection process described in paragraph 9.9.8.2 of the cited draftstandard, and for use in the bit allocation process described inparagraph 9.9.14 of the cited draft standard, but in addition is veryuseful for estimating certain transmission channel characteristics suchas the position of a bridged tap along the line. A preferred embodimentof the present invention consequently makes use of this automaticallycollected information for transmission channel characterisation.

Thus, by multiplexing the collected information that will be used fortransmission channel characterisation with user data, a transmissionchannel can be characterised without interruption of the system. This isso because the remote terminal that is able to transmit the user datadoes not have to be replaced by test equipment.

Alternatively, a transmission channel characteristic is estimatedoff-line.

Thus, by interrupting transfer of user data over the channel andbringing the system into a test phase, a transmission channelcharacteristic according to the present invention can also be estimatedoff-line without the requirement to connect dedicated test equipment tothe channel.

In this way, information indicative for the attenuation and phaserotation of carriers having different frequencies, such as is typicallycollected in a multi-carrier ADSL (Asymmetric Digital Subscriber Line)modem and used therein for setting equaliser coefficients, canadditionally be used for channel characterisation.

In this way, information indicative for the noise affecting carriershaving different frequencies, such as is typically collected in amulti-carrier ADSL modem and used therein for allocation of bits tocarriers, can additionally be used for channel characterisation.

Furthermore, the present invention is very suitable for implementationin an Asymmetric Digital Subscriber Line (ADSL) system, or any similarsystem, e.g. a Very High Speed Digital Subscriber Line (VDSL) system.

An additional advantageous feature of the arrangement for estimating atransmission channel characteristic.

Hence, information suitable for use in the transmission channelcharacterisation method can be collected over a certain time interval,and can be fed back to the central office as soon as this time intervalhas elapsed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned features of the invention will become more apparentand the invention itself will be best understood by referring to thefollowing description of an embodiment taken in conjunction with theaccompanying drawings:

FIG. 1 is a functional block scheme of a telecommunication systemwherein the method to estimate a transmission channel characteristicaccording to the present invention is applied; and

FIG. 2 is a flowchart of the method to estimate a transmission channelcharacteristic.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the central office CO 1 and remote terminal RT 4 of anAsymmetric Digital Subscriber Line (ADSL) system are drawn. The remoteterminal RT 4 in addition to traditional ADSL receiving circuitry thatis supposed to form part of the receiver RX 5 and traditional ADSLtransmitting circuitry that is supposed to form part of the transmitterTX 6 contains a channel information gathering unit INFO 7 and channelinformation memory MEM 8. The central office CO 1 on top of traditionalADSL transceiving circuitry, not drawn in the figure, contains a channeland modem matching unit MATCH 2.

The receiver RX 5 in the remote terminal RT 4 is coupled between aninput/output port and an output terminal of this remote terminal RT 4,and further has an output coupled to the channel information gatheringunit INFO 7. The transmitter TX 6 in the remote terminal RT 4 is coupledbetween an input terminal and the above mentioned input/output port ofthe remote terminal RT 4. To a second input terminal of the transmitterTX 6, the channel information memory MEM 8 is coupled. An output of thechannel information gathering unit INFO 7 and an input of the channelinformation memory MEM 8 are interconnected. A twisted pair telephoneline CHANNEL 3 serves as bi-directional physical communication mediumbetween the remote terminal RT 4 and the central office CO 1. The remoteterminal RT 4 is coupled to this telephone line CHANNEL 3 via the abovementioned input/output port. In the central office CO 1, an input of thechannel and modem matching unit MATCH 2 is coupled to the telephone lineCHANNEL 3.

In the drawn ADSL system digital data are transferred bi-directionallyon top of telephone signals over the telephone line CHANNEL 3. At boththe remote side and the central office side, a splitter separatesreceived digital data from received telephone signals, applies thedigital data to respectively the ADSL remote terminal RT 4 or ADSLcentral office CO 1, and applies the telephone signals to respectivelythe customer's telephone apparatus or a POTS (Plain Old TelephoneService) line-card. In FIG. 1, the splitters at the remote side andcentral office side, and the customer's telephone apparatus as well asthe POTS line-card are not drawn because the present invention only hasan impact on the ADSL central office CO 1 and the ADSL remote terminalRT 4.

For the operator of the drawn system, one of the problems is how toguarantee a certain service to the customer at the time of installationor later without exact knowledge of the channel characteristics of thetelephone line CHANNEL 3. Because of the frequent changes indisturbances (e.g. the presence of a radio amateur) and line faults(e.g. due to water logging), databases containing information concerningthe channel characteristics of telephone lines usually are outdated.Since the remote terminal RT 4 of the drawn ADSL system automaticallycollects information that is useful for channel characteristicinformation, access to this information by the operator enables theoperator to track changes in the channel characteristics.

The remote terminal RT 4 in FIG. 1 is supposed to operate in accordancewith Issue 2 of ANSI's T1E1.413 standard and consequently performscertain channel measurements such as a gain and phase measurement foreach carrier transferred over the telephone line CHANNEL 3 and a signalto noise ratio (SNR) measurement for each carrier. The so obtainedinformation is used for setting parameters of a time domain equaliserand frequency domain equaliser which typically forms part of the ADSLreceiver RX 5 and for allocating data bits to carriers in the ADSLtransmitter TX 6. Referring to FIG. 2, at S100, the channel informationgathering unit INFO 7 collects the measured information and theinformation is temporarily stored in the channel information memory MEM8. Thereafter, at S200, the ADSL transmitter TX 6 reads the channelinformation out of the channel information memory MEM 8 and sends theinformation to the central office CO 1 so that it is available at theoperator's side for channel estimation purposes.

At S300, in the central office CO 1, the channel and modem matching unitMATCH 2 use a simulation model of the telephone line CHANNEL 3 andremote terminal RT 4 and matches the received information with thissimulation model. The simulation model contains a number of parameterslike the line attenuation, the length of the telephone line, length andlocation of bridged taps, water logging, and so on. By matching thesimulation model with the information received from the remote terminalRT 4, at S400, the channel and modem matching unit MATCH 2 determinesthe values for the different parameters in the simulation model. Theseparameter values are an estimation of the channel characteristics wherethe operator can rely on to guarantee a certain service to the customer.

Since the measurements can be done by the remote terminal RT 4 on-line,the channel characteristics can be monitored in time so that each timethe customer wants to install a new service, recent channelcharacteristic estimations are available to the operator allowing theoperator to give quality guarantees concerning the newly installedservice. The permanently available channel characteristic estimationsmoreover enable the operator to take appropriate measures in case ofquality loss, to monitor changes to the customer's in-house network thatmay affect the quality of service, and to fast evaluate complaints ofthe customer concerning the quality of service.

The channel information gathering unit INFO 7, that is coupled to anoutput terminal of the ADSL receiver RX 5 in the above describedembodiment, may be integrated within this ADSL receiver RX 5. Indeed,since an ADSL receiver RX 5 that operates in accordance with thespecifications of ANSI's T1E1.413 Issue 2 standard collects channelinformation at initialisation for use in the bit allocation algorithmand for setting equaliser taps, channel information gathering equipmentINFO 7 may in certain embodiments of the invention be integrated in thereceiver circuitry RX 5.

Although the embodiment of the present invention illustrated by FIG. 1contains a channel information memory MEM 8, provision of such a channelinformation memory MEM 8 is not an absolute requirement forimplementation of the present invention. Indeed, without the channelinformation memory MEM 8, the channel information collected by theremote terminal RT 4 would be fed back instantly to the central officeCO 1, whereas with a channel information memory MEM 8, the channelinformation can be collected during a certain time interval and can befed back at certain time instants in between the time intervals whereinthe channel information is collected. When providing a channelinformation memory MEM 8, the collected channel information may be sentautomatically at regular time intervals to the central office CO 1 ormay be sent to the central office CO upon request of the latter one.

The channel and remote terminal matching unit MATCH 2 may form part ofthe central office CO 1, as is the case for the embodiment of theinvention drawn in the figure, but alternatively may be a separate unitconnected to an operator terminal of the central office CO 1.

The applicability of the present invention is not limited to anyparticular physical communication medium in between the central officeCO 1 and the remote terminal RT 4 or any particular physical layertransport protocol. Thus, instead of a twisted pair telephone line, acoaxial cable, an optical fibre, a radio link, a satellite connection,or even a hybrid transmission medium such as a hybrid coax/optical fibrelink may serve as communication medium between the central office CO 1and remote terminal RT 4, and evidently, instead of the AsymmetricDigital Subscriber Line (ADSL) protocol, any other physical layerprotocol, e.g. the VDSL (Very High Speed Digital Subscriber Line)protocol or the Hybrid Fibre Coax (HFC) protocol may be adopted in thesystem wherein the present invention is implemented.

Furthermore, the embodiments of the present invention are describedabove in terms of functional blocks. From the functional description ofthese blocks it will be obvious for a person skilled in the art ofdesigning electronic devices how embodiments of these blocks can bemanufactured with well-known electronic components. A detailedarchitecture of the contents of the functional blocks hence is not givenfor most of them.

While the principles of the invention have been described above inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationon the scope of the invention.

1. A method to estimate a transmission channel characteristic of atransmission channel between a central office and a remote terminal,said method comprising the steps of: a. collecting at said remoteterminal channel measurements performed at a number of frequencies; b.transferring said channel measurements from said remote terminal to saidcentral office; and c. estimating said channel characteristic on thebasis of said transferred channel measurements.
 2. A remote terminalsuitable for communication with a central office via a transmissionchannel therebetween, said terminal having a channel informationgathering component for collecting channel measurements performed at anumber of frequencies, said remote terminal further including a channelmeasurement transmission component which transfers said channelmeasurements to said central office so that said channel measurementsare available at said central office for the purpose of estimating achannel characteristic of said transmission channel.
 3. A transmissionchannel characteristic estimating unit for estimating a transmissionchannel characteristic of a transmission channel between a centraloffice and a remote terminal, said unit comprising a channelcharacteristic estimating component which estimates said channelcharacteristic on the basis of channel measurements performed at anumber of frequencies and received from said remote terminal.